Fluid handling components, such as channels, pipes, tubes and associated fittings and other components have been used for millennia to convey liquids from one place or process to another. Friction of moving fluids with the fluid handling components, however, has always presented a significant challenge to achieving maximum efficiency in fluid handling systems. Friction increases the energy required to pump a fluid through a system and reduces the fluid flow rate through the system.
It is known that the physical characteristics of the fluid contact surfaces of fluid handling components have an effect on friction of the fluid with the components. Generally, for example, smoother surfaces reduce friction, while rougher surfaces increase friction. Also, surfaces made from materials resistant to wetting, such as PTFE, exhibit relatively lower fluid friction. Surfaces that are resistant to wetting by liquids are referred to as “phobic” surfaces. Such surfaces may be known as hydrophobic where the liquid is water, and lyophobic relative to other liquids.
Previous attempts at reducing fluid friction in fluid handling systems have been only partially successful. While fluid friction may be reduced by providing smoother fluid contact surfaces, the amount of reduction achievable is limited. Likewise, the use of conventional materials with improved surface wetting characteristics, such as PTFE, may result in some improvement in friction properties, but the amount of improvement is limited. Also, the choice of materials may be restricted based on the compatibility of the fluid with the materials to be used.
Some recent work has focused on developing special “ultraphobic” surfaces for use in fluid handling applications, particularly in microfluidic applications. Generally, if a surface resists wetting to an extent that a small droplet of water or other liquid exhibits a very high stationary contact angle with the surface (greater than about 120 degrees), if the surface exhibits a markedly reduced propensity to retain liquid droplets, or if a liquid-gas-solid interface exists at the surface when completely submerged in liquid, the surface may be referred to as an ultrahydrophobic or ultralyophobic surface. For the purposes of this application, the term ultraphobic is used to refer generally to both ultrahydrophobic and ultralyophobic surfaces.
Friction between a liquid and a surface may be dramatically lower for an ultraphobic surface as opposed to a conventional surface. As a result, ultraphobic surfaces are extremely desirable for reducing surface friction and increasing flow in a myriad of hydraulic and hydrodynamic applications on a macro scale, and especially in microfluidic applications.
It is now well known that surface roughness has a significant effect on the degree of surface wetting. It has been generally observed that, under some circumstances, roughness can cause liquid to adhere more strongly to the surface than to a corresponding smooth surface. Under other circumstances, however, roughness may cause the liquid to adhere less strongly to the rough surface than the smooth surface. In some circumstances, the surface may be ultraphobic.
Efforts have been made previously at introducing intentional roughness on a surface to produce an ultraphobic surface. The roughened surface generally takes the form of a substrate member with a multiplicity of microscale to nanoscale projections or cavities, referred to herein as “asperities”.
Previous attempts at producing ultraphobic surfaces with micro/nanoscale asperities have been only partially successful. Generally, while the prior art surfaces have exhibited ultraphobic properties under some circumstances relative to liquid droplets carefully placed on the surface, the properties generally disappear when a droplet is impacted with the surface or the surface is submerged in liquid.
Moreover, fluid pressure in fluid handling applications where ultraphobic surfaces may be desirably used often exceeds one atmosphere, and in extreme applications, may reach hundreds of atmospheres. Ultraphobic surfaces produced to date appear to be effective as an ultraphobic surface only up to about 0.1 atmospheres, severely limiting the applicability of such surfaces in fluid handling component applications.
In addition, prior art ultraphobic surfaces are often formed with delicate polymer or chemical coatings deposited on the substrate. These coatings are easily physically damaged, even by fluid pressure, so as to be ineffective. Fluid handing component applications typically require durable fluid contact surfaces so that the component has a reasonable effective life span.
Drainability is also often an important characteristic in fluid handling systems. It is typically necessary to drain most fluid handling systems at some time, whether for maintenance or other reasons. For a variety of reasons, it is generally desirable that as much of the fluid as possible be drained from the system at such times. Moreover, it may be critical that substantially all of the fluid is drained from the system in applications such as semiconductor processing in order to minimize undesirable process contamination.
Often, in conventional fluid handling systems, there is sufficient adhesion between the fluid and the fluid contact surfaces so that individual fluid droplets adhere to the fluid contact surfaces in the system. These droplets are not easily removed, and in a large system, may include a substantial quantity of fluid.
Still needed in the industry are fluid handling components that provide significantly reduced fluid friction characteristics at pressure, combined with improved drainability characteristics.